GABA, 4-aminobutyric acid, is the primary inhibitory transmitter in the brain and maintains a balance between excitation and inhibition of neurons. Three major classes of GABA receptors have been identified: GABAA, GABAB and GABAC receptors. GABAA and GABAC receptors are ligand-gated ion channels (LGIC), while GABAB receptors are G-protein coupled receptors. The LGIC receptors are heteropentamers comprised of α1-6, β1-3, γ1-3, ρ1-3, δ, ε, π, and θ subunits. Each subunit contains four membrane-spanning domains. The N-terminal domain and C-domain are extracellular and the agonist/antagonist binding site is situated on the N-terminus. There is an intracellular loop between the 3rd and 4th membrane spanning regions (M. Chabib and G. A. R. Johnston, J. Med. Chem. 2000 43(8):1427-1447).
While studies are continuing to define the composition and anatomical distribution of GABA LGIC receptors, it is known that the dominant motif is 2α2β1γ with varying α subtypes. Subtype assemblies containing an α1 subunit are present in most areas of the brain and are thought to account for over 40% of GABAA receptors in the rat. Subtype assemblies containing α2β2/3γ2 and α3βnγ2/3 oligomers are thought to account for about 18% and 17% respectively of GABAA receptors in the rat (R. M. McKeman et al. Trend Neurosci 1996 19:139-143). Subtype assemblies containing an α5 subunit are expressed predominantly in the hippocampus and cortex and are thought to represent about 4% of GABAA receptors in the rat. The most common receptor subtype assemblies appear to be the α1β2γ2, α2β3γ2, α3β3γ2 and α5β3γ2 assemblies (H. Mohler et al. Neuroch. Res. 1995 20(5):631-636).
All known GABAA receptors contain a plurality of distinct modulatory sites, one of which is the benzodiazepine (BZ) binding site. Other modulatory sites include allosteric sites for picrotoxin, barbiturates, neuroactive steroids and ethanol. The BZ binding site is the most explored of the GABAA receptor modulatory sites, and is the site through which anxiolytic drugs such as diazepam exert their effect. Early radioligand binding studies suggested the existence of two distinct benzodiazepine-binding sites: BZ1 and BZ2. The BZ1 subtype has been shown to be pharmacologically equivalent to a GABAA receptor comprising the α1 subunit in combination with a β subunit and γ2. This is the most abundant GABAA receptor subtype. Two other major populations are the α2βγ2 and α3β2/3γ2 subtypes. Together these constitute approximately a further 35% of the total GABAA receptor repertoire. Pharmacologically, the α2βγ2 and α3β2/3γ2 subtypes appear to be equivalent to the BZ2 subtype. The physiological role of these subtypes has hitherto been unclear because sufficiently selective agonists or antagonists were unknown.
The barbiturates and benzodiazepines were among the first clinically useful modulators of the GABA receptors and are among the most widely prescribed medications for anxiety, depression and other psychiatric disorders and as anticonvulsants. Benzodiazepines, with relatively mild side effects, afforded an alternative to barbiturates which possess more potent side effects. Unfortunately, many of the early benzodiazepines had relatively limited subtype selectivity resulting in sedation, dependence, cognitive impairment, ataxia, potentiation of ethanol effects, tolerance and withdrawal.
The advances in genetics and molecular biology have afforded more subtle probes of receptor subtype selectivity and hold out the promise of more selective agents. Receptors containing the α1, α2, α3 or α5 subunit have been classified as diazepam sensitive receptors while α4, or α6, are classified as diazepam insensitive receptors. In particular, the α1 subtype has been associated with sedation and α1 selective ligands have potential as sedatives (R. M. McKernan et al. Nature Neurosci. 2000 3(6): 587-592). Hypnotic/sedative compounds with preferential binding for the α1 subtype have been identified (D. J. Sanger and H. Depoortere, CNS Drug Reviews, 1998 47(5):323-340). Sedation, however, is undesirable in an anxiolytic agent.
Compounds that selectively bind to the benzodiazepine site, or to other allosteric sites, and enhance the ability of GABA to open GABAA receptor channels are agonists (or positive allosteric modulators) of GABA receptors. Compounds that interact with allosteric sites but negatively modulate the action of GABA are called inverse agonists (negative allosteric modulators). Inverse agonists diminish the ability of GABA to open receptor channels. A third class of compounds that bind selectively to the benzodiazepine site and yet have little or no effect on GABA activity, but can block the action of GABAA receptor agonists or inverse agonists that act at this site are referred to as antagonists. Agonists that act at the benzodiazepine site exhibit anxiolytic, sedative, and hypnotic effects, while compounds that act as inverse agonists at this site elicit anxiogenic, cognition enhancing, and proconvulsant effects.
The α1 selective GABAA receptor agonists alpidem and zolpidem are clinically prescribed as hypnotic agents, further suggesting that at least some of the sedation associated with known anxiolytic drugs is mediated through GABAA receptors containing the α1 subunit. Accordingly, GABAA receptor agonists which interact more selectively with the α2 and/or the α3 subunit relative to the α1 subunit should be effective anxiolytics with a reduced propensity to cause sedation. Also, agents which are antagonists or inverse agonists at the α1 subtype might antagonize sedation or hypnosis caused by α1 modulators.
Selective α2 and α3 ligands have been more difficult to identify and cross-reactivity between these receptors is common. Compounds with ten to one hundred-fold selectivity for α2/3 relative to α1 have been reported (see, e.g., W. R. Carling et al., WO 0044752). Experiments with point mutated mice lines suggest that the α2, not the α3, subtype is responsible for the anxiolytic activity (U. Rudolph et al. Trends Pharmacol. Sci.2001 22(4):188-194; K. Löw et al. Science 2000 290:131-134); however, α3-selective inverse agonists appear to be anxiogenic and proconvulsant (I. J. Collins et al. WO 9855480). Since α2 and perhaps α3 selective ligands have the potential to modulate the (BZ2) site without activating the hypnotic sedative site (BZ1) they could afford a new class of non-sedating anxiolytics. Other non-BZ selective α2 GABA modulators may also exhibit anxiolytic properties without many of unwanted effects.
The selective ligands for GABAA receptors of the present invention are useful in the treatment and/or prevention of anxiety disorders, such as panic disorder with or without agoraphobia, agoraphobia without history of panic disorder, phobias including social phobias, obsessive-compulsive disorder, stress disorders including post-traumatic and acute stress disorder, and generalized or substance-induced anxiety disorder; depression or bipolar disorders such as single-episode or recurrent major depressive disorder, dysthymic disorder, bipolar I and bipolar II manic disorders; schizophrenia; learning and cognitive disorder such as Alzheimer's disease and attention deficit hyperactivity disorder; sleep disorders and disorders of circadian rhythm, e.g. in subjects suffering from the effects of jet lag or shift work; convulsive or seizure disorders such as epilepsy and pain.
Other neurotransmitter systems have been explored and drugs modulating serotonergic neurotransmission have shown promise in the treatment of anxiety related disorders. Recently, drugs such as buspirone, a partial agonist at 5HT1A receptor, and serotonin reuptake inhibitors, commonly used as antidepressants, have been introduced. GABAA selective ligands may potentiate the effects of certain other CNS active compounds. There is evidence that selective serotonin reuptake inhibitors (SSRIs) show greater antidepressant activity when used in combination with GABAA selective ligands than when used alone.